Replication processes for producing plastic optical components

ABSTRACT

Polymeric replica molds formed from certain specific polymeric compositions are disclosed. The polymeric compositions impart properties to the replica molds which allow the casting of high-quality plastic optical components such as ophthalmic lenses. 
     Among the polymeric compositions which have been found useful are: (1) cross-linked polymethyl methacrylate; (2) a copolymer of 99-20 parts methyl methacrylate and 1-80 parts acrylonitrile, preferably but not necessarily cross-linked; (3) cross-linked polystyrene; (4) a cross-linked copolymer of 90-10 parts styrene and 10-90 parts acrylonitrile; and, (5) a cross-linked copolymer of 90-10 parts styrene and 10-90 parts methyl methacrylate. 
     Replication processes including the steps of forming the polymeric replica molds described herein and subsequently using them in the casting of plastic lenses from materials such as polymerized allyl diglycol carbonate are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division, of application Ser. No. 210,519, filed Dec. 21,1971, now U.S. Pat. No. 3,830,460 which in turn, is acontinuation-in-part of application Ser. No. 841,758, filed July 15,1969, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to the casting of plastic opticalcomponents, and more particularly to replication processes for castingplastic lenses using replica molds formed from polymeric compositionswhich impart properties to the replica molds necessary to casthigh-quality ophthalmic lenses.

2. Description of the Prior Art

Replication molding techniques are well known. In such a technique, areplica mold having a surface which is a negative copy of a surface ofthe final product is first prepared from a master mold. The finalproduct is then molded against the negative surface of the replica mold,thus reproducing the surface configuration of the master mold.

Attempts have been made to manufacture plastic optical elements,particularly plastic ophthalmic lenses, by replication techniques, butsatisfactory results have not heretofore been obtained. Exemplary ofsuch attempts are the processes taught in U.S. Pat. Nos. 3,422,168 and3,423,488, both to G. H. Bowser. These patents teach a replicationprocess in which plastic lenses are cast from replica molds formed frommaterials such as low molecular weight, non-emulsifiable polyethyleneresins. It has been found, however, that plastic lenses cast usingBowser type polyethylene replica molds invariably contain certaindefects including minute surface imperfections which cannot beeliminated from the polyethylene replica mold surfaces. Some of thesesurface imperfections include scratch-like markings, surface blemishes,graying, orange peel, etc. Additionally, Bowser-type polyethylenereplica molds have high degrees of flexibility, and because of this,distortion of the plastic lens occurs during cure which makes itimpossible to reproduce consistent lens powers from one casting to thenext with the accuracy required by ophthalmic standards. Lens distortionis a particular problem when polyethylene replica molds are used to casthigh power lenses, but even plano (zero power) lenses cannot be cast toany degree of repeatable accuracy with these materials.

The criticality of the polymeric compositions used to form replica moldshas been confirmed by many unsuccessful attempts to obtain accuratereplications of glass surfaces using a multitude of commerciallyavailable resins to form the replica molds. It has been found that thegreat majority of these resins do not work satisfactorily for one reasonor another. Some resins tried unsuccessfully include polymers of allylmethacrylate, styrene triallyl cyanurate, diethylene glycoldimethacrylate, triethylene glycol dimethacrylate, and even polymerizedallyl diglycol carbonate itself as a replica mold material.

It is believed that there are many reasons why most polymericcompositions tried do not yield satisfactory results in replicationprocesses such as those described herein. For example, low-meltingpolyethylenes, such as those taught by Bowser, appear to formmicroscopic defects in the mold surfaces which reproduce in the finalplastic lenses. Also, these materials are not believed to besufficiently rigid and stiff to prevent distortion of the curing plasticlens. On the other hand, materials such as linear polymethylmethacrylate produce replica molds whose surfaces are solvent attackedby one of the materials primarily used in forming plastic lenses, i.e.allyl diglycol carbonate resin (sold commercially by Pittsburgh PlateGlass Co. under the registered trademark CR-39). A solvent attack ofthis nature produces a matte finish on the final plastic lens making itunsatisfactory.

Another property which makes some polymeric materials unacceptable istheir release characteristics from the plastic lens being molded.Examples of materials which do not have satisfactory releasecharacteristics from cured CR-39 include such resins as linearpolystyrene, polyester, alkyd and epoxy.

Despite the fact that the attempts to date have been unsuccessful inarriving at a replication process using polymeric replica molds whichproduces high-quality optical components and which is commerciallyfeasible, there is a tremendous amount of research continuing in thisarea for such a process. The many great advantages costwise andotherwise which would flow from such a successful process are welldocumented in the literature in general and in particular in theabove-mentioned Bowser patents.

SUMMARY OF AN EMBODIMENT OF THE INVENTION

This invention relates to the use of a limited number of polymericcompositions to form replica molds useful in replication molding ofplastic optical components. The new polymeric compositions are thosewhich provide the replica molds with characteristics necessary toproduce high-quality plastic optical components from CR-39 and otherplastics. These characteristics include: (a) good releasability from amaster mold surface which is usually, but not necessarily, formed fromglass or metal; (b) balanced adhesion to the plastic optical elementduring and after its cure; (c) resistance to solvent attack by theplastic optical element during its cure; (d) thermal stability at themolding temperature; and, (e) the capability of forming optical qualitymold surfaces which are reproducible in the molded plastic opticalelement.

As mentioned above, it has been found that most common polymericcompositions do not provide replica molds having these properties. Fivepolymeric compositions which have been found to provide the desiredcharacteristics include: (1) polymethyl methacrylate cross-linked withminor amounts of a cross-linking agent; (2) a copolymer of 99-20 partsmethyl methacrylate and 1-80 parts of acrylonitrile wherein thecopolymer is linear or preferably cross-linked with a minor amount of across-linking agent; (3) polystyrene cross-linked with a minor amount ofa cross-linking agent; (4) a copolymer of 90-10 parts styrene and 10-90parts of acrylonitrile wherein the copolymer is cross-linked with aminor amount of a cross-linking agent; and, (5) a copolymer of 90-10parts styrene and 10-90 parts methyl methacrylate cross-linked with aminor amount of a cross-linking agent.

It should be noted that it is not possible to specify broad classes ofpolymers by their chemical characteristics with any degree ofassuredness that all species in the broad class will form suitablereplica molds. Therefore, the description herein, in contrast to much ofthe prior art, will not attempt to describe the polymeric compositionsfound suitable by using such labels as "thermosetting," "thermoplastic,""linear," "cross-linked," "homopolymers," "copolymers," etc. It will beseen that various polymeric compositions have been found suitable whichfit into all of these classes. On the other hand, it has been found thatthe polymeric compositions must have certain characteristics during andafter cure of the plastic lens being molded, and it is possible todefine suitable polymers in terms of these characteristics.

Replication molding techniques for producing plastic optical elementsfrom CR-39 and other plastic resins can be used with the new replicamolds. A replica mold with a surface configuration negative to that ofthe final element is formed from polymeric compositions such as thosedescribed above. The plastic resin is then molded against the polymericreplica mold, and after cure the plastic optical element is releasedfrom the replica mold. This procedure can be repeated using the samereplica mold so that a plurality of lenses can be made from one replicamold.

The polymeric replica molds and replication processes described hereinhave many advantages over those previously known. For example, thereplica molds described herein can be repeatedly used to produce manyplastic lenses if care is taken to keep them clean. This was not true ofmany of the prior art replica molds such as those taught in the Bowserpatents since lowmelting polyethylenes melted and tended to fusetogether during casting of the final plastic lens. The concomitanteconomic advantages are clear.

The most important advantage, however, is the quality of the plasticoptical elements that can be cast using the replica molds andreplication processes described herein. The quality is so high, in fact,that the plastic lenses produced are suitable for use in ophthalmicapplications. These lenses have less defects in their surfaces thanprior art lenses on both a macroscopic and microscopic level.Furthermore, the lenses produced are substantially free from distortion,and plastic lenses of even high powers are consistently reproducible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustratess a vertical section cut through a casting assemblyincluding a glass master lens surface and a plastic resin from which areplica mold having a concave surface can be formed;

FIG. 2 illustrates a casting assembly within a pressure chamber whereinthe casting assembly is suitable for forming a convex replica mold froma concave glass master lens;

FIG. 3 illustrates the casting of a plastic lens between two replicamolds.

FIGS. 4a through 4e show shearing interferograms of plastic lensesproduced according to the prior art.

FIGS. 5a through 5e show shearing interferograms of plastic lensesproduced as described herein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the replication of plastic lenses, the replica mold surface is formedagainst a master mold surface. Usually the master mold is formed fromglass or metal, but other materials could be used as well. Therefore, animportant characteristic which the polymeric compositions used mustimpart to the replica molds is releasability from a master mold surfaceafter the replica mold composition has cured. Polymeric compositionswhich do not have good releasability from the master mold surface arenot suitable for replica molds.

The polymeric compositions used to form the replica molds must alsoexhibit balanced adhesion to the plastic optical element during itscure. Since some plastics, such as CR-39, shrink during their cure, thereplica mold must exhibit sufficient adhesion to follow the shrinkage ofthe plastic lens during cure. Otherwise, the replica mold will becomeseparated from the plastic lens prior to the completion of cure whichresults in lens distortion. On the other hand, after curing iscompleted, the replica mold must release from the plastic lens withoutcausing surface defects in the lens. Balanced adhesion can be obtainedby matching the polymeric composition of the replica mold to theparticular plastic from which the lens is being formed. Those skilled inthe art will know suitable materials or be able to match materials usingonly routine experimentation.

Another characteristic which the polymeric composition must impart tothe replica mold is resistance to solvent attack by the plasticcomposition from which the lens is being formed. It is important in thisregard to recognize that both the lens monomer and polymer must notattack the replica mold. If either does, surface imperfections will becaused in the mold surface which carry through to the final lenssurfaces. Resistance to solvent attack is something that can beroutinely checked in the various handbooks available to those skilled inthe art or can be determined by no more than routine experimentation.

Thermal stability at the molding temperature for the plastic opticalelement is another requirement for the polymeric compositions. With someplastic compositions, such as CR-39, there is some leeway in the curingtemperatures. For example, it has been found that CR-39 can be cured at50°C. in 3 days, whereas cycles reaching higher temperatures, such as115°C., can shorten the cure cycle to less than 1 day. In general, it isan advantage to have the replica molds thermally stable at highertemperatures so that the cure cycle can be shortened. The polymericcompositions should, therefore, be thermally stable at 50°C., andpreferably 115°C.

To prevent distortion of the plastic element during cure, it has beenfound that the polymeric composition forming the replica mold must havea moderate to high degree of rigidity, or stiffness. Although, aspointed out above, the material must have the capability to follow theelement's shrinkage during cure, it has been found that highly flexiblematerials, such as the low molecular weight polyethylenes described inBowser, result in intolerable amounts of lens distortion. Sufficientmold rigidity and stiffness can be obtained by making the moldssufficiently thick and from plastic compositions having a flexuralmodulus above about 2 × 10⁵ psi, and preferably above about 3.5 × 10⁵psi. For this invention, a material's flexural modulus can be determinedby A.S.T.M. test D790. Values for the flexural modulus of variouspolymeric materials are given in most plastic handbooks including ModernPlastics Encyclopedia, 1970-71, vol. 47, No. 10A, October, 1970.

Replica molds for producing plastic ophthalmic lenses according to thisinvention usually have a thickness ranging between 3 and 25 millimeters.The most preferred thicknesses are 6-15 millimeters. Thinner polymericcompositions can be used if they are suitably backed by a rigid member.

It has been found that polymeric compositions which provide replicamolds with the above-mentioned characteristics are indeed unique. Aspointed out above, many polymeric compositions have been found to beunsuccessful because they did not provide one or more of the abovecharacteristics. On the other hand, five polymeric compositions,representative of those which do provide the required characteristics,are now presented.

The first of these materials is polymethyl methacrylate cross-linkedwith a minor amount of a cross-linking agent. Preferably, the polymethylmethacrylate is cross-linked with from about 5 to about 30% of across-linking agent.

Another material found to produce suitable replica molds is a copolymerformed from 99-20 parts of methyl methacrylate and 1-80 parts ofacrylonitrile. It is not necessary to cross-link this copolymer,although minor amounts of a cross-linking agent are preferable. In amore preferred embodiment, this copolymer contains 65-25 parts methylmethacrylate and 35-75 parts acrylonitrile, and the copolymer iscross-linked with about 5-30% of a cross-linking agent. It has beenfound that an outstanding replica mold can be produced from a 30/70copolymer of methyl methacrylate/acrylonitrile cross-linked about 2 to15%, which is the most preferred embodiment.

A further polymeric composition suitable for replica molds is styrenecross-linked with minor amounts of a cross-linking agent. In a morepreferred case, the polystyrene is cross-linked with from 1 to 20% of across-linking agent.

Another suitable polymeric composition is a copolymer formed from 90-10parts of styrene and 10-90 parts of acrylonitrile, with the copolymerbeing cross-linked with minor amounts of a cross-linking agent. In amore preferred embodiment, this copolymer comprises 80-60 parts ofstyrene and 20-40 parts of acrylonitrile, cross-linked with 5-20% of across-linking agent.

A further suitable polymeric composition is a copolymer formed from90-10 parts styrene and 10-90 parts methyl methacrylate cross-linkedwith 5-30% of a cross-linking agent. In a more preferred embodiment,this copolymer is comprised of 80-30 parts styrene and 20-70 partsmethyl methacrylate with about 10-20% cross-linking agent. The mostpreferred copolymer comprises about 50 parts of both styrene and methylmethacrylate with about 10% cross-linking agent.

Although most of the polymers recited above would be generallycategorized as vinyl or acrylic polymers, it is important to recognizethat many other types of polymers will produce polymeric compositionsproviding replica molds with the necessary characteristics. As can alsobe seen, some of the materials have been found to require cross-linkingwhile others do not. The critical distinction between polymericcompositions suitable for producing replica molds as described hereinand those of the prior art is that the newly described polymericcompositions will produce a replica mold having the characteristicsspecified above. The polymeric compositions heretofore used forophthalmic replica molds do not have these characteristics, and becauseof that they have not resulted in replication processes which producehigh-quality optical elements on a commercial basis.

Some of the preferred polymeric compositions set out above arecross-linked. Suitable cross-linking agents are those monomers whichpossess a functionality exceeding two, i.e. polyfunctional. Thoseskilled in the art will known many suitable cross-linking agents. Thepreferred cross-linking agents are glycol dimethacrylates which can beproduced by esterifying methacrylic acid with a glycol containing from 2to 4 carbon atoms or with a polyglycol having from 4 to 8 carbon atoms.Such glycols include diethylene glycol, triethylene glycol,tetraethylene glycol, dipropylene glycol and the like as well asethylene glycol and propylene glycol. These materials are hereinafterdescribed simply as glycol dimethacrylates.

Another preferred cross-linking agent is divinylbenzene. Divinylbenzeneis obtainable commercially as a 55% solution, with the major impuritiesbeing believed to be ethyl vinyl benzene, diethylbenzene and a varietyof non-volatile components. This mix can be used as received, or it canbe purified by fractionation to give divinylbenzene of better than 95%purity. The purified divinylbenzene is preferred.

Additional cross-linking agents which are suitable include allylmethacrylate, diethylene glycol dimethacrylate, triethylene glycoldimethacrylate, tetraethylene glycol dimethacrylate, triethylol propanetrimethacrylate, glyceryl trimethacrylate, pentaerythritoltetramethacrylate and the corresponding acrylates. Others includetriallyl cyanurate, diallyl phthalate, diallyl adipate, diallylsuccinate, diallyl maleate, diallyl fumarate, and diallyl diglycolate.

In this description, the amount of cross-linking agent used is specifiedin terms of weight percent of the polymer or copolymer.

Replication processes for producing high-quality plastic opticalelements using the hereindescribed replica molds are carried out asfollows. In the first step, a replica mold is formed which has a surfaceconfiguration negative to a predetermined surface configuration for theplastic optical element. The replica mold is formed from a plasticcomposition such as those described above by casting against the mastermold's positive surface. After a replica mold has been formed, asuitable plastic, such as CR-39, is then molded against the replica moldsurface configuration. After cure, the plastic optical element isreleased from the replica mold.

It is possible to cast the polymeric compositions against the mastermolds described herein at atmospheric pressure. In most cases, however,it is preferred to use a positive pressure to hold the replica moldsagainst the masters. This is particularly true where curved surfaces orsurfaces having discontinuities are to be reproduced. Pressuresufficient to compensate for shrinkage of the resin during casting ispreferably used, and in most cases pressures in the order of 50 lbs. persquare inch gage or higher produce the best results.

To a limited extent, the molds can be case between two rigid surfaces,provided they are allowed to come together by means of a peripheralflexible gasket. Much better results are usually obtainable, however,when the replica mold is cast between a rigid surface which carries thecurvature of the ophthalmic lens or other optical surface to bereproduced, and a flexible member capable of accomodating thepolymerization shrinkage of the resin. Such a member may be made of anymaterial having the necessary flexibility and mechanical strength. Onevery suitable member comprises a sheet of aluminum foil from about 0.003inches to 0.040 inches thick. Such a foil can be formed to any desiredshape by spinning, stamping or other conventional means.

Most plastic lenses to date have been manufactured from CR-39(polymerized allyl diglycol carbonate). It should be clear, however,that while the plastic lenses produced as described herein arepreferably CR-39, many other plastics can also be used. Some examples ofother materials include unsaturated alcohol esters of simple polybasicacids such as diallyl phthalate, diallyl maleate, diallyl fumarate,diallyl succinate, diallyl carbonate, diallyl crotonate, diallyldiglycolate, and any other of the many resins that will copolymerizewith CR-39 resin, such as dimethallyl phthalate, vinyl acetate, andmethyl methacrylate. Other suitable plastic compositions include thoseknown as TEMA resins which are produced by American Cyanamid and areusually substantially water-insoluble, non-gelled, unsaturated polyesterresin compositions such as those described in U.S. Pat. Nos. 3,265,763;3,364,291 and 3,457,104, the teachings of which are hereby incorporatedby reference. Other resins having the required optical and moldingproperties could, of course, be used.

The replication processes will now be described in detail by referenceto the Figures.

In FIG. 1, a glass master lens 1, having a convex surface 2 to bereproduced, is mounted in a holder 3 having a shoulder 4. A cup-shapedflexible member 5, which can be spun aluminum, has a central web 6terminating in a side wall 7 and an outer flange 8 adapted to make asliding fit with the shoulder 4. The flange 8 preferably has a lateralsurface 9 adapted to engage the surface 10 of a clamping ring 11.Lateral surface 9 and flange 8 can be painted with a vinyl plastisolwhich is subsequently primed and baked on to insure a tight seal.

An annular plate 15 engages holder 3 above shoulder 4 and the assemblyis clamped together by bolts 16 and 17 having ends 18 and 19 adapted forthreaded engagement with nuts 13 on the opposite sides of the ring 11.Preferably these bolts pass through openings 20 in the plate 15. Whenthe bolts 16 and 17 are tightened, a positive pressure of 50-100 lbs.per square inch or more can be applied to the casting after the surface10 has engaged the lateral surface 9 of the membrane 5.

In the apparatus of FIG. 2, a glass master lens 25 is mounted in aholder 26 having a shoulder 27. The lens is shown as having a concavesurface 28 to be reproduced, although it should be evident that theconvex surface 29 could be substituted simply by reversing the positionof the lens on shoulder 27, as shown in FIG. 1. The outer edge of thelens is preferably covered with a fillet 26a of sealing cement to make aliquid-tight joint.

A flexible, aluminum foil member 30 is formed having a central web 31terminating in a side wall 32 and an outer flange 33 adapted to make asliding fit with the shoulder 27. Member 30 and lens holder 26 arereceived in rings 34 and 35, which are clamped together by bolts 36.

Polymerization shrinkage of the plastic composition being molded isaccommodated by the movement of the flexible member, and thus largecurvatures and sharp surfaces discontinuities can be reproducedaccurately.

In order to apply positive pressure to the casting, the entire mold isenclosed within an outer pressure-resistant casing 40 adapted with aclosely fitting lid 41 affixed by clamps, bolts or other securing meansthat are not shown. An opening 42 is provided to permit the applicationof pneumatic or hydrostatic pressure to the flexible member 30 andthrough flexure of this member to the plastic composition during thecuring thereof. When both positive pressure and a flexible member areused in this manner, one in conjunction with the other, it is possibleto cast highly complex shapes and extreme curvatures and discontinuitiesin the mold surfaces and the like from any of the resins describedabove. If desired, a second opening 43 may be provided in the side wallof the casing 40 to permit continuous flow of the pressurizing fluids.

Castings are carried out in this equipment by placing a predeterminedquantity of one of the above-described polymeric compositions in anessentially monomeric or partially polymerized but still liquidcondition into the cavity 12 of flexible member 5 in FIG. 1, or into themold cavity 45 of FIG. 2. The cavity is closed and the temperature iscontrolled by the medium circulating in casing 40 until polymerizationis complete. Curing catalysts such as azo bis-isobutyrlonitrile,peroxides, percarbonates and the like may be included in the mannercustomary in molding resins of this type. Polymerization is generallycarried out at temperatures within the range of about 40°C. to about95°C. or slightly higher and for times that will depend on the thicknessof the casting and other related factors. Suitable curing times areusually within the range of about 1 to 6 hours but may be as high as upto 35-40 hours with unusually thick castings.

It is an advantage in the embodiment of FIG. 2 that both heat andpressure may be applied through the same medium. Thus, water or airheated to the desired curing temperature and under suitable pressure,preferably about 50 to 100 pounds per square inch, may be introducedthrough the inlet opening 42 and applied to the member 30 andtransmitted by its flexure to the plastic resin in cavity 45. Thus, theflexible member 30 serves three purposes: (1) it acts as a mold cavitywall; (2) it serves to accomodate lens shrinkage; and (3) it serves as agood conductor of heat to the curing lens.

Continuous positively controlled temperature regulation can be obtainedby maintaining a flow of such fluids through inlet opening 42 and theoutlet opening 43, and the curing cycle can be greatly shortened bycirculating a temperature controlled fluid in this manner. This willpermit a more rapid transfer of heat out of the casting for thosepolymeric compositions exhibiting exotherms as they polymerize.

The replica molds prepared in this manner are used to cast plasticresins, such as CR-39, into optical components and particularlyophthalmic lenses. A representative assembly illustrating their use forthis purpose is shown in FIG. 3. Castings 12 and 45 are the castingsindicated by the same reference numerals in FIGS. 1 and 2, respectively.These are accommodated in opposition by an annular spacing gasket 46. Ameasured quantity of the plastic resin mix, such as CR-39 monomer, isintroduced into the space 47 between castings 12 and 45 together withone of the catalysts described in U.S. Pat. No. 2,384,115, if desired.The casting assembly is held together by clamps. Ring 46 is preferablymade of a flexible material such as plasticized vinyl resin orpolyethylene-polyisobutylene. The assembly is then heated to curingtemperatures, preferably within the range of about 40°C. to 90°C., untilthe plastic resin has polymerized to the desired extent. After cooling,the plastic lens is released from between the replica molds.

In the description above, the term "plastic optical element" has beenused. While in most cases, it is used in connection with plasticophthalmic lenses, it should be understood that the term is intended tomean any type of plastic element which has an optical quality surfacethereon which includes lenses other than ophthalmic lens, mirrors, etc.,and even includes plastic molds for forming optical components.

The following examples illustrate the remarkable improvement which canbe achieved using polymeric replica molds and processes as describedherein instead of those described in the prior art. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE I Prior Art Replication Process

The materials and procedure of Example I, U.S. Pat. No. 3,422,168, werefollowed with minor differences as noted to produce plastic lenses bythe most well known prior art technique.

Two ground and polished glass lenses having a convex surface of sixdiopters and a concave surface of six diopters and having a diameter of66 millimeters were used as the master lenses. The resin selected toconstitute the finished plastic lens was allyl diglycol carbonate(CR-39). A catalyst and drying agent were added, and the resin wasfiltered and subsequently stored under refrigeration as described in thesecond paragraph of Example I, U.S. Pat. No. 3,422,168.

The master glass lenses were mounted in aluminum mounting rings completewith rubber gaskets and bolted covers through which pressure could beintroduced. In one case, the convex side of one lens formed the bottomof a cavity in the aluminum mounting ring; in the other, the concaveside of the lens formed the bottom of a similar cavity. The cavities inthe mounting rings were filled with a low-melting polyethylene resin("Epolene" C-15 was used since "Epolene" C-12 is no longer commerciallyavailable) which had been heated to 257°F. The resin was poured alongthe edge of the mounting ring so as not to pour it directly onto themaster glass lenses.

The rubber gaskets were inserted and the covers for the mounting ringsbolted down. The assemblies were allowed to air cool to 130°F., and thecovers and gaskets were removed. The convex and concave replica moldsformed were simply pryed from the aluminum mounting rings.

The two replica molds were then used to mold CR-39 lenses. Thepreviously prepared and refrigerated CR-39 resin was poured directlyinto the depression of the concave mold and the convex portion was theninserted. The replica molds filled with CR-39 resin were passed throughthe following curing cycle in an air circulating oven to cure the CR-39.

    ______________________________________                                        Time (hrs:min)       Temp. °F.                                         ______________________________________                                        0:00                 115                                                      1:00                 118                                                      2:00                 122                                                      3:00                 126                                                      4:00                 129                                                      5:00                 145                                                      5:30                 151                                                      5:45                 154                                                      5:55                 159                                                      6:10                 165                                                      6:30                 176                                                      7:30                 End of cycle                                             ______________________________________                                    

The replica molds were separated by hand and the finished CR-39 lens wasremoved.

Five CR-39 lenses were produced following the above-describedprocedures. These lenses had surface imperfections such as pin pricks,scratch-like markings or surface blemishes which were visible to theeye; also, some matching imperfections could be seen in the replica moldsurfaces. Additionally, it could be observed that the lenses were allbadly distorted.

To further evaluate the lenses, a telescope test was used to determinetheir refractive power and definition. For plano lenses, the testconditions are referred to as USA Standard Z87.1--1968, published by theIndustrial Safety Equipment Association, Inc., as approved Sept. 18,1968 by the USA Standards Institute. These methods are equivalent to theNational Bureau of Standards methods which are as follows:

Refractive Power

The lenses may be tested for refractive power with an eight-powertelescope which has an effective aperture of 0.75 inch and is focused ata distance of 35 feet on an illuminated test chart. The resolving-powerchart pattern 20 of National Bureau of Standards circulus C533 may beused. The lens to be tested shall be placed in front of the telescopeobjective which is then brought to the sharpest possible focus. Thepattern marked 20 should be clearly resolved with the target placed at adistance of 35 feet from telescope objective used for testing lenses.The telescope is calibrated by successively locating the position ofbest focus with first a standard lens of plus 1/16 diopter in front ofthe objective and then with a standard lens of minus 1/16 diopter infront of the objective. These positions are marked by scratches on thedraw tube or by other suitable index marks. The lens is to be held infront of the calibrated telescope and if the position of the best focusfalls outside the index marks, the refractive power is in excess of 1/16diopter.

Definition

The lenses may be tested for definition with an eight-power telescopewhich has an effective aperture of 0.75 inch and is focused at adistance of 35 feet on an illuminated test chart. As a test chart theresolving-power chart pattern 20 of National Bureau of StandardsCircular C533 may be used. The lens to be tested shall be placed infront of the telescope objective, which in turn is then brought to thesharpest possible focus. The pattern marked 20 should be clearlyresolved with the target placed at a distance of 35 feet from thetelescope objective used for testing lenses.

All five lenses produced by following the procedures of this Exampleusing low-melting polyethylene replica molds failed the above-describedtelescope test.

Shearing interferograms of five lenses produced by this method weretaken in transmission. A lateral shearing interferometer was used whichwas set up as follows. The lens being tested was placed in a divergingbeam of light such that the light leaving the lens was essentiallycollimated. The collimated beam was incident on a plane parallel plateof glass. Part of the collimated beam was reflected from the frontsurface of the glass, and part of it was reflected from the backsurface. The two reflected beams were displaced laterally (sheared) adistance S. If the beam leaving the lens was not perfectly collimated,interference fringes appeared in the common area of the two reflectedbeams. The spacing of these fringes has been shown to be inverselyproportional to the relative local power of the lens. A more detailedexplanation of this shearing interferometric technique for testingophthalmic lenses is presented in: Murty, M. V. R. K.; Applied Optics;Apr., 1964; page 531.

The interferograms are presented in FIG. 4. They are a graphicalrepresentation of surface power variation and defects in the testedlenses. Perfectly vertical and equally spaced interference linesrepresent the most uniform power distribution over the lens surfaces.

As can be seen from the shearing interferograms of FIG. 4, both thepower variation and surface smoothness of the five lenses was poor. Thepower variation in lenses 4a and 4e were particularly objectionable andwere easily discerned with the unaided eye. Other particularlyobjectionable features were the high frequency fringes or local defectsnoticed in lenses 4b and 4e. These local defects were replicas of pitsand dents in the low density polyethylene molds.

The same five lenses were also viewed under an 80X microscope. All hadwell-defined areas containing a high density of very fine parallelscratches approximately 0.01 inches long. These areas were easily seenwhen the lenses were examined under a bright light. The low densitypolyethylene molds used to make these lenses possessed correspondingscratch containing areas.

EXAMPLE II Replication Process Using Replica Molds Formed from PolymericCompositions as Described Herein

The glass master was the same as in Example I. A master was used in anapparatus as shown in FIG. 1 to produce a replica mold having a concavesurface; another master was used in an apparatus of FIG. 2 to produce areplica mold having a convex surface.

The replica molds were formed from cross-linked polystyrene. The liquidresin cast against the glass masters contained:

    styrene monomer            90 parts                                           ethylene glycol dimethacrylate                                                                           10 parts                                           isopropyl percarbonate     0.05%                                              benzoyl peroxide           0.05%                                              t-butyl perbenzoate        0.05%                                              cobalt octoate (12% solution in dibutyl                                                                  0.02%                                              phthalate)                                                                    Zelec UN (a mold-releasing agent believed to                                                             0.05%                                              be a lauroyl phosphate sold by DuPont).                                   

The percents given are weight percents based on the copolymer.

Curing of the styrene resin was done in an autoclave as illustrated inFIG. 2 under a positive pressure of 60 psi maintained by circulatingwater. The water temperature was held at 60°C. for 4 hours, then raisedto 115°C. for 2 hours and finally allowed to cool to 70°C.

After curing, the pressure was released and the mold was opened. Thealuminum foil was peeled off and the plastic replica molds separatedeasily from the glass master. Each replica mold was 9 millimeters inthickness.

CR-39 resin, prepared and refrigerated as in Example I, was introducedinto the cavity formed when the replica molds were fitted together. Thecure cycle of Example I was also used for the CR-39 resin. After curethe CR-39 lenses were easily released from the cross-linked polystyrenereplica molds.

Five plastic lenses produced according to this Example were examined.These plastic lenses had dramatically fewer imperfections visible to theeye and much less distortion. They were used in the telescope test andall passed.

Shearing interferograms were taken of five lenses produced following theprocedure of this Example. The results are presented in FIG. 5.

As can be seen, the power variation and surface smoothness are vastlyimproved over those qualities in the lenses produced in Example I.

Additionally, the areas containing a high density of very fine parallelscratches seen under an 80X microscope with the lenses of Example I arenot present on the lenses produced according to Example II.

From the tests performed, it can be seen that the lenses produced byExample II are significantly improved over the lenses produced by theprocedure of Example I. The lenses produced in Example II are suitablefor ophthalmic uses, whereas the lenses produced in Example I are not.

Reference is made to my commonly assigned copending applications, Ser.No. 210,488, filed Dec. 21, 1971, now U.S. Pat. No. 3,806,079, and Ser.No. 210,518, filed Dec. 21, 1971, the teachings of which are herebyincorporated by reference.

What is claimed is:
 1. A process for producing a plastic optical elementhaving a predetermined surface configuration comprising:a. forming areplica mold having a surface configuration negative to said element'spredetermined surface configuration, said negative surface configurationbeing formed from a polymeric composition which has a flexural modulusabove about 2 × 10⁵ psi so that the mold will resist distortion of theplastic optical element during cure thereof, said polymeric compositionalso being one which: (1) is releasable from a master mold surface; (2)has balanced adhesion to said plastic optical element during curethereof; (3) is resistant to solvent attack by said plastic opticalelement during cure thereof; (4) is thermally stable at the moldingtemperature for said plastic optical element; and (5) is capable offorming an optical quality surface on the plastic optical element; b.molding said plastic optical element against said replica mold surfaceconfiguration; and, c. releasing said optical element after cure thereoffrom said replica mold.
 2. A process of claim 1 wherein said polymericcomposition is releasable from a glass master mold surface.
 3. A processof claim 2 wherein said plastic optical element comprises an ophthalmiclens.
 4. A process of claim 3 wherein said ophthalmic lens is formedfrom polymerized allyl diglycol carbonate.
 5. A process of claim 4wherein the molding of said plastic optical element is conducted at apositive pressure of from about 50 to about 100 psi.
 6. A process ofclaim 5 wherein the molding of said plastic optical element is conductedat a temperature of from about 40°C. to about 95°C.
 7. A process ofclaim 6 wherein a flexible member is used to absorb polymerizationshrinkage during molding of said plastic replica mold.
 8. A process ofclaim 7 wherein said polymeric composition comprises a member selectedfrom the group consisting of:a. polymethyl methacrylate cross-linkedwith a minor amount of a cross-linking agent; b. a copolymer formed fromabout 99 to about 20 parts of methyl methacrylate and from about 1 toabout 80 parts of acrylonitrile, said copolymer being cross-linked withfrom 0 to a minor amount of a cross-linking agent; c. polystyrenecross-linked with a minor amount of a cross-linking agent; d. acopolymer formed from about 90 to 10 parts of styrene and from about 10to about 90 parts of acrylonitrile, said copolymer being cross-linkedwith a minor amount of a cross-linking agent; and, e. a copolymer formedfrom about 90 to about 10 parts of styrene and from about 10 to about 90parts of methyl methacrylate, said copolymer being cross-linked with aminor amount of a cross-linking agent.
 9. A process of claim 7 whereinsaid polymeric composition comprises polystyrene cross-linked with fromabout 1 to about 20% of a cross-linking agent.
 10. A process of claim 9wherein said cross-linking agent comprises glycol dimethacrylate.
 11. Aprocess of claim 7 wherein said polymeric composition is a copolymercomprising from about 65 parts to about 25 parts of methyl methacrylateand from about 35 parts to about 75 parts of acrylonitrile, saidcopolymer being cross-linked with from about 5% to about 30% of across-linking agent.
 12. A process of claim 7 wherein said polymericcomposition is a copolymer comprising about 30 parts of methylmethacrylate and about 70 parts of acrylonitrile, said copolymer beingcross-linked with from about 5 to about 30% of glycol dimethacrylate.13. A process for forming a plurality of ophthalmic lenses having apredetermined surface configuration from polymerized allyl diglycolcarbonate comprising:a. forming a glass master mold having a surfaceconfiguration corresponding to said predetermined surface configuration;b. casting a copolymer comprising from about 65 to about 25 parts ofmethyl methacrylate and from about 35 to about 75 parts ofacrylonitrile, said copolymer being cross-linked with from about 5% toabout 30% of a cross-linking agent, against said master mold to form areplica mold having a surface configuration negative to that of saidmaster mold; c. casting allyl diglycol carbonate against said replicamold at a temperature of from about 40°C. to about 95°C. and a pressureof from about 50 to about 100 psi to form an ophthalmic lens; d.releasing said ophthalmic lens from said replica mold; and, e. repeatingsteps c and d.
 14. In a replication process for producing ophthalmiclenses wherein a replica mold is formed by casting a polymeric resinagainst a glass master and subsequently casting an ophthalmic resinagainst said replica mold to produce ophthalmic lenses:the improvementcomprising selecting a polymeric resin which has a flexural modulusabove about 2 × 10⁵ psi so that the mold will resist distortion of theophthalmic lens during cure thereof and which has a composition of:a.polymethyl methacralate cross-linked with from about 5 to about 30% of across-linking agent; or, b. a copolymer formed from about 65 to about 25parts methyl methacralate, from about 35 to about 75 partsacrylonitrile, and from about 5 to about 30% of a cross-linking agent;or, c. polystyrene cross-linked with from about 1 to about 20% of across-linking agent; or, d. a copolymer of from about 80 to about 60parts of styrene, from about 20 to about 40 parts of acrylonitrile, andfrom about 5 to about 20% of a cross-linking agent; or, e. a copolymerof from about 80 to about 30 parts of styrene, from about 20 to about 70parts of methyl methacralate and from about 10 to about 20% of across-linking agent.